Array instance offset variance
I don't know how to do that and keep it in an array modifier, but here's a workaround: Drop the array and duplicate the tile in the Z direction. Select the tiles you want to randomize, then go to Object > Transform > Randomize Transform. The controls will appear in the toolshelf T.

Also:

There was some activity toward improving the existing array modifier in this way a few years ago, but it never got to the trunk.

Here's the story on Blender Nation: http://www.blendernation.com/2012/01/05/advanced-array-modifier/

And here is a build for it on GraphicAll: http://graphicall.org/537

You could model with that, apply the modifier, then continue working in your preferred version.